For representing struct field default values and array/pointer type
sentinel values, we use `*const anyopaque`, since there is no way for
`std.builtin.Type.StructField` etc to refer back to its `type` field.
However, when introspecting a type, this is quite awkward due to the
pointer casts necessary.
As such, this commit renames the `sentinel` fields to `sentinel_ptr`,
and the `default_value` field to `default_value_ptr`, and introduces
helper methods `sentinel()` and `defaultValue()` to load the values.
These methods are marked as `inline` because their return value, which
is always comptime-known, is very often required at comptime by use
sites, so this avoids having to annotate such calls with `comptime`.
This is a breaking change, although note that 0.14.0 is already a
breaking release for all users of `std.builtin.Type` due to the union
fields being renamed.
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